The present invention relates to an improved tantalum-carbon hybrid capacitor.
A capacitor is typically made of two parallel surfaces or plates, each of which is an electrode. The electrodes are separated by a dielectric, that is, an insulating material. The dielectric may be a gas, liquid, solid, or vacuum. The capacitance is directly proportional to the surface area of the electrodes and inversely proportional to the thickness of the dielectric.
An electrolytic capacitor consists of a series combination of two conductors (e.g., foil electrodes or plates) at least one of which is a valve metal. A dielectric film is formed adjacent to the surface of one or both of the electrodes. The dielectric is an oxide that has a high dielectric constant and a high dielectric strength: it is electrochemically deposited in very thin layers, and its thickness is usually on the order of hundreds to tens of thousands of Angstroms (xc3x85).
An electrolytic capacitor uses an ionically conductive chemical compound or xe2x80x9celectrolyte,xe2x80x9d in between its electrodes (i.e., metallic surfaces of plates). The electrolyte provides a large area for ionic charges to collect near the dielectric layer of the electrode(s), thereby greatly increasing the capacitance or the ability to store energy as electrostatic charges. The high volumetric efficiency of an electrolytic capacitor is due to its enhanced plate surface area and a very thin dielectric layer.
Electrolytic capacitors may be either xe2x80x9cdry,xe2x80x9d or xe2x80x9cwet.xe2x80x9d A dry electrolytic capacitor uses non-aqueous electrolyte that has very low water content. A dry electrolytic capacitor employs a separator between the electrodes: the separator is saturated wit the electrolyte.
A wet electrolytic capacitor uses a liquid or aqueous electrolyte, and therefore employs a liquid-tight container. An anode is submerged in the electrolyte inside the container. Although anode and cathode foils could be rolled together with a separator and submerged in the electrolyte, the container is normally attached to and therefore part of the cathode and a discrete anode is submerged in the electrolyte.
Tantalum-carbon wet electrolytic capacitors are useful in a number of applications, for example, as filters, for energy storage, for low source impedance circuits, for high charging-current circuits, for switching regulators supplies, and for other purposes.
Tantalum-carbon capacitors provide higher volumetric efficiency than pure wet tantalum-capacitors. The latter type of capacitor has a tantalum oxide anode and a tantalum oxide cathode. The higher efficiency of a tantalum-carbon capacitor is achieved by making the cathode out of a thin material with a high surface area. By doing so, a larger anode may be used in the same case size. The inner surface of similar capacitors have formerly been coated with a porous oxide of a metal such as platinum, ruthenium, iridium, nickel, rhodium, platinum, palladium, or osmium. See, for example, U.S. Pat. Nos. 5,982,609, 5,737,181, 5,559,677, 5,469,325, 5,469,547, 5,098,485, 4,942,500, and 4,780,797. The coating increases the available surface area, and therefore the capacitance. Such metals provide large surface area because of their small particle size and physical structure, and provide good conductivity. However, their cost is very high.
In the invention, activated carbon is used in place of the metals mentioned above. Activated carbon provides extremely high surface area and good conductivity. High capacitance is achieved, but without the cost associated with the use of platinum or other similar metals.